The present invention generally relates to the forming of tubular structural members by internal pressurization of the member, and more particularly to such method as applied to a tubular structural member with a varying diameter.
Hydroforming methods are commonly known as a means for shaping a tubular metal blank having a circular cross section into a tubular component having a predetermined desired configuration. A typical hydroforming operation involves the placement of a tubular metal blank having a circular, uniform cross section into a die cavity of a hydroforming assembly and providing high pressure fluid to the interior of the blank to cause the blank to expand outwardly into conformity with the surfaces defining the die cavity. Typically, the opposite longitudinal ends of the tubular metal blank are sealed by hydraulic rams, and high pressure hydroforming fluid is provided through a port formed in at least one of the rams to expand the tubular blank.
The cross-sectional shape of the resultant structural member is typically appreciably different from the cross-sectional shape of the tube blank. For example, box shaped or quadrilateral cross sections are commonly formed from the cylindrical tube blanks.
In relatively low-pressure hydroforming, the forming is essentially limited to pressurizing the tube blank to force it to conform to an enclosing die, and produces little change in the length of a part. The length of the formed part is essentially equal to the length of the tube blank. There is generally a tradeoff made between section perimeter and the local wall thickness. Hydroforming will generally increase perimeter and decrease wall thickness.
It is appreciated that a variant form of low-pressure hydroforming utilizes extremely high pressure and produces parts shorter than the tube blank to provide an increase in perimeter with little or no associated decrease in wall thickness. This requires a longitudinal loading of the blank simultaneous with the pressurization of the blank.
It is often highly desirable to produce parts having wall perimeters that vary locally along the length of the formed part by more than the amount available with convention hydroforming, such as a tubular member with variations of greater than 100% in diameter or perimeter with respect to the smallest diameter or perimeter with respect to the smallest diameter or perimeter of the member. Unfortunately, hydroforming of a conventional tube blank does not readily lend itself to providing a part with such localized variations due to excessive thinning of the wall thickness at the areas of greatest required expansion.
One approach to hydroforming parts that satisfy longitudinally varying thickness and perimeter requirements is to form a tailored tube blank that has a non-uniform perimeter and/or thickness adapted to the structural member to be hydroformed, as disclosed in U.S. Pat. No. 5,333,775. Here, tube blanks having different wall thicknesses and/or perimeters are welded together by conventional means to form a compound tube blank.
In this patent, two tube blanks of varying diameter are joined by first forming a transition region between them so that the two adjacent ends meet. These transition regions consist of either a tapered or a flared end being formed on the end of either or both blanks, which is formed using conventional tube forming means, such as with tube expanders. Alternatively, an entirely separate piece of tubing with a truncated conical shape can be used as a transition blank member between two blanks of different diameters. In this case, the diameter of each end of the transition member corresponds to the tube blank end that it interfaces with. In either case, conventional welding is used to join the blanks in end-to-end abutting relation. The resultant tube blank is then hydroformed. The die into which the blank is placed has a generally corresponding transition perimeter to that of the tube blank so that the expansion of the perimeter is limited to approximately five percent in any one location along a longitudinal axis of the blank.
The method described in U.S. Pat. No. 5,333,775 requires the inconvenient steps of first tapering or flaring the blank ends using tube expanders, or inserting a separate, conical shaped tube blank between two blanks with different diameters to form a transition region connecting them, and then tediously welding the ends together in end-to-end fashion. In addition, while an end-to-end weld may be satisfactory for certain applications, it may not be optimal for more demanding hydroforming requirements.
It is an object of the present invention, therefore, to overcome the problems associated with the prior art noted above. In achieving this object, the present invention provides a method of hydroforming a tubular part which comprises providing a first tubular member having a first diameter, providing a second tubular member having a second diameter which is greater than the first diameter of the first tubular member, positioning the first and second tubular members so that a first portion of the first tubular member is telescopically disposed within a second portion of the second tubular member so that the first and second portions are disposed in axially overlapping relation with one another, applying an electromagnetic pulse of sufficient magnitude in a vicinity of the overlapping portions to rapidly force the first and second portions into peripheral welded engagement with one another so as to form a welded tubular structure, placing the welded tubular structure into a hydroforming die assembly, and providing fluid pressure to the interior thereof so that it expands outwardly into conformity with the die surfaces of the hydroforming die assembly.
The electromagnet pulse can be applied either from the exterior of the tubular assembly; in which case the second outer tubular member is forced to move inwardly and into welded contact with the first tubular member, or the electromagnetic pulse can be applied from the interior of the tubular assembly, in which case the first inner tubular member is forced to move outwardly and into welded contact with the second tubular member. Alternatively, each of the first and second members can be moved towards the other.